Company I work for issues a basic data structures test to be completed in 1 hour for all resumes worth their paper (about 10 to 15 out of 100 candidates). The test boils down to 'fill in these two functions'. About 20% finish both functions correctly in an hour (I still had an infinite recursion at the 1 hour mark and submitted a correct entry 10 minutes later).

In conjunction with a phone interview we typically only have an actual sitdown with 2 to 4 candidates out of the initial pool of 100. We recruit out of college so resumes get handed in like candy, but this process has effectively helped us find candidates that are at least capable of learning the monolithic code base and getting fully up and running in a few months.

Our company still has an absolute terrible code base. It is very feasible to nuke skilled programmers once you have them.

This only works if there's a time-limit. Facebook's doesn't have one, so they make the problems inordinately time-consuming, which has the side-effect of filtering for not just skill, but desire to work at Facebook as opposed to the many alternatives that won't make you spend the equivalent of nearly an entire workday applying. Six months ago when I applied, I spent a day completing a puzzle--and getting their puzzle-bot to accept it... it only runs every few hours, on input which is unbounded so you're not sure how optimized your solution has to be. If your solution doesn't pass, the puzzle-bot does not tell you why. Afterwards they asked me to do another puzzle at which point I voluntarily dropped out of their interview process. Overall I would say the experience was very poor and reflected badly on Facebook in my mind.

Second most important thing after judging whether you could handle being stuck in a hot office with the guy for a month.